Applicant claims priority under 35 U.S.C. xc2xa7119 of Austrian Application No. A 936/2000 filed May 29, 2000. Applicant also claims priority under 35 U.S.C. xc2xa7365 of PCT/ATOl/00169 filed May 29, 2001. The international application under PCT article 21(2) was not published in English.
The invention relates to a Stirling engine comprising at least one working piston and at least one displacement piston.
Depending on what type of drive unit is provided for a rotary drive, there are many possible ways of controlling the power of the rotary drive. In combustion engines, the power can be controlled very well via the fuel supply, whereas, e.g., in Stirling engines a power control without a loss in efficiency has been a great problem for quite some time. For controlling the power of Stirling engines it has been known to change the clearance volumes, on the one hand, and to change the pressure of the working gas, on the other hand, wherein, however, losses in efficiency, or relatively long reduction times, respectively, occur with both types of power control.
From U.S. Pat. No. 3,886,744 A, e.g., a power control system for a Stirling engine is known in which the inlet pressure of the hot air is controlled via an annular control element which opens or closes the inlet depending on the differential pressure present; this has the disadvantage that a very complex construction is provided and that the efficiency of the Stirling engine declines as a consequence of the pressure control.
From U.S. Pat. No. 2,873,611 A, a combustion engine is knownxe2x80x94in which the stroke of a piston can be changed with the assistance of a circular-arc-shaped lever arm, and thus the power of the driven-side crank can be adjusted. For this purpose, the lever arm has a connecting link guide in which a connecting head is slidably mounted. Since, however, in combustion engines numerous other advantageous possibilities are available for an efficient power control, such an arrangement is not suitable in combustion engines.
The invention has as its object to provide a Stirling engine of the initially defined type with which a rapid power control is possible without lowering its efficiency.
The Stirling engine according to the invention and of the initially defined type is characterized in that for a power control by means of the transmission of the linear movement of a drive part into the linear movement of a driven part, a lever articulately connected to the drive part and the driven part is provided, which lever has an associated displaceable pivot point, the bearing point of the lever traveling on the pivot point according to a curve during the movement transmission. This curve may have any shape desiredxe2x80x94depending on the requirements of the movement transmission and on the type of the respective Stirling engine.
Since the theoretical power of a Stirling enginexe2x80x94considering an isothermal expansion and compressionxe2x80x94can be expressed by       P    =                  (                  1          -          τ                )            ⁢                        π          *          n                60            ⁢              V                  E          ,          max                    *              P        m            ⁢              δ                  1          +                                    1              -                              δ                2                                                        *      sin      ⁢              xe2x80x83            ⁢      Θ        ,  with
P . . . power
xcfx84. . . temperature ratio between compression space and expansion space,
n . . . number of revolutions [U/min]
VE,max . . . maximum volume of the expansion space
Vc,max . . . maximum volume of the compression space
Pm . . . effective mean pressure
xcex4. . . pressure ratio of engine and             Θ      ⁢              xe2x80x83            ⁢      …      ⁢              xe2x80x83            ⁢      tan      ⁢              xe2x80x83            ⁢      Θ        =                  w        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢        ϕ                    τ        +                  w          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          ϕ                      ,
with xcfx86=phase angle between working piston and displacement piston, and   w  =            V              C        ,        max                    V              E        ,        max            
the ratio of the maximum volumes of compression and expansion, as well as   τ  =            T      C              T      E      
the temperature ratio between compression volume and expansion volume, a power control can be carried out by means of the lever arrangement according to the characterizing part of claim 1 without any losses in efficiency, since preferably the maximum compression volume Vc,max and, thus, the pressure ratios xcex4 of the engine can be controlled very well.
By adjusting the pivot point on which the lever or its bearing point, respectively, travel during the movement transmission, thus the velocity and the acceleration of the driven part and a change in the maximum volumes of the compression space caused thereby can be obtained in a very simple way, whereby the power of the Stirling engine can be controlled.
For realizing the change of the bearing point of the lever during the movement transmission with a simple construction, it is advantageous if the lever has a connecting link defining the given curve, which connecting link slides over the pivot point, e.g. via a roller defining this pivot point, during the movement transmission.
For a well-defined power control of the Stirling engine it has proven particularly advantageous if the curve or connecting link has the shape of a circular arc; yet other curve shapes are, of course, also conceivable for certain purposes of use, e.g. two tangentially connected circular arc segments, or an elliptical shape.
To allow for a simple adjustment of the pivot point, it is advantageous if the pivot point is arranged on a pivot arm.
Shifting of the pivot point can be realized in a structurally particularly simple manner if the pivot arm is connected to an adjustment device.
To equally adjust each point of rotation of two leversxe2x80x94in case at least two cylinders are used, it is advantageous if the adjustment device is connected to a pivot arm via one linkage each and is symmetrically provided between at least two levers.
For a simple configuration of the adjustment device in terms of construction, it is suitable if a spindle drive is provided as the adjustment device.
If a connecting link guide is provided in which the end of the linkage arranged opposite the pivot arm is displaceably and fixably received, the position of the pivot arm can be changed in a simple and quick manner and thus, the power of the Stirling engine can be adjusted.
In a Stirling engine with a double-active working cylinder, in which the movement of the working piston occurs in a sine-shape, it is advantageous if the displacement piston is associated with the lever for a power control, whereby a dynamic stroke change as well as a discontinuous movement of the displacement piston will occur.
In a xcex2-Stirling engine, with which in general higher mechanic efficiencies are obtained than with the remaining types of Stirling engines, the displacement piston and the working piston are located in a common cylinder, whereby, in theory, it is possible for the entire gas mass to be located in the hot space during the expansion phase and to be located in the cold space during the compression phase. For an efficiency-neutral power control it is then advantageous if the working piston is associated with the lever with a displaceable pivot point, and the displacement lever is associated with a lever with a non-displaceable pivot point.
In a double-active engine, in which the working piston and the displacement piston form one unit for the purpose of a simple construction of the Stirling engine, this unit is associated with the lever for an advantageous power control.
For a reliable movement of the displacement piston and of the working piston, respectively, it is suitable if the drive part is articulately connected to a piston rod which is linearly guided in a straight-line guide and connected to the displacement piston or to the working piston, respectively.
For the required heat exchange to the working gas between the heater and cooler surfaces, respectively, it is suitable if the displacement piston on both sides and the working piston on one side thereof has a wave-shaped section capable of engaging in the neighboring heater or cooler surfaces, respectively. In this manner, substantially larger surfaces can get into contact with the working gas, as compared to plane surfaces. As regards a high strength of the displacement piston, it is suitable if the lamella-type wave-shaped sections of the displacement piston are arranged to be turned by 90xc2x0 relative to each other. Also for a high strength it is advantageous if the lamella-type thin-walled wave-shaped sections of the working piston or heater head, respectively, are supported by stiffening ribs at the burner side and at the coolant side, respectively. An integration of heater-, regenerator- and cooler-surfaces directly into the working space is particularly advantageous in terms of efficiency and of minimizing the detrimental volume of a Stirling engine.
Instead of cooperating at the driven side with a conventional crankshaft, it may be advantageous in terms of the kinematics for a maximum approach to the ideal circle process, if the linear movement of the driven part is converted into a rotational movement by means of a connecting link which serves as a crank.